The production of cycloalkylaromatic compounds, such as cyclohexylbenzene, is a commercially important reaction since the latter has potential as a source of phenol and cyclohexanone, which are important products in the chemical industry with utility in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the demand for propylene is likely to increase. Thus, a process that does not require propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol.
One such process involves the catalytic hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513, in which the hydroalkylation catalyst is a bifunctional catalyst comprising at least one hydrogenation metal component and a molecular sieve of the MCM-22 family.
However, one problem of producing phenol via benzene hydroalkylation is that the hydroalkylation process inevitably produces significant quantities of by-products in addition to the desired cyclohexylbenzene. Among the more significant of these by-products are cyclohexane, dicyclohexylbenzene, dicyclohexane and methylcyclopentylbenzenes. Although many of these impurities can be removed by down-stream processing steps, such steps necessarily add cost to the overall process and hence there is significant interest in improving the cyclohexylbenzene selectivity of the hydroalkylation process.
Following extensive study of the hydroalkylation reaction, it is now believed that one mechanism for the production of unwanted by-products is the slow mass transport of reagents and products since the process is normally conducted in the liquid phase at low temperature. Another potential source of by-product formation is believed to be reactive intermediates, such as cyclohexene, since the latter can readily isomerize to produce methylcyclopentene and dimerize in the presence of hydrogen to produce dicyclohexane.
According to the present invention, it has now been found that the amount of by-products generated in the hydroalkylation reaction can be reduced by the addition of a diluent to the reaction. Although the reason for this result is not fully understood, it is believed that the diluent improves mass transport by facilitating movement of the cyclohexylbenzene away from the active sites of the catalyst and hence minimizing subsequent reactions such as dialkylation. In addition, the use of a diluent will reduce the steady state concentration of reactive intermediates, such as cyclohexene, thereby reducing side reactions involving these intermediates, such as isomerization and dimerization. Finally, the addition of a diluent is believed to assist in removal of heat of reaction, thereby ensuring more homogeneous heat distribution and hence improving cyclohexylbenzene selectivity.
U.S. Pat. No. 3,786,106 discloses a process for reacting an aromatic compound, such as benzene, with a cycloolefin, such as cyclohexene, to produce a cycloalkylaromatic compound, such as cyclohexylbenzene, over an active clay catalyst. In addition, the '106 patent refers to adding a diluent to the process in the form a straight or branched chain paraffinic hydrocarbon having 5 to 10 carbon atoms. However, the function and amount of the diluent is not disclosed or explained, and the basic process is alkylation rather than hydroalkylation in the presence of hydrogen.